Key Message

This indicator and its threshold values are yet to be commonly agreed in HELCOM.The indictor is included as a test indicator for the purposes of the 'State of the Baltic Sea' report, and the results are to be considered as intermediate.

A provisional status evaluation has been done for specific assessment units over the period 2011–2016 to test the application of this indicator (Key message figure 1). The threshold values, based on defined reference periods, assess acceptable deviations from seasonal growth curves of dominating phytoplankton groups. The indictor value is based on the number of data points which fall within the acceptable deviation range, as set for each monthly phase in the reference growth curve, and expressed as the percentage to the total number of observations. Strong deviations from the reference growth curves indicate impairment in the environmental status.

The proportion of observations with acceptable deviations in monthly biomass indicating normal seasonal succession of phytoplankton, was preliminarily set at ≥0.67 for the tested areas. To develop basin-specific threshold values, all data were analysed to detect periods with lower total biomass and lesser year-to-year fluctuations. The analysis for 12 assessment units resulted in threshold values varying from 0.58 to 0.74 (Thresholds and Status evaluation table 2).

Where applied, the confidence in this indicator assessment is moderate or high. This indicator should be applicable in all coastal and open sea waters around the Baltic Sea. To account for spatial differences in phytoplankton community composition and environmental gradients, further development and agreement related to appropriate threshold value setting is required.

Relevance of the core indicator

Phytoplankton are the key primary producers in marine ecosystems. The phytoplankton community is comprised of several functionally diverse groups that dominate at different times of the year. Changes in the presence of specific phytoplankton group or the timing of when it dominates and becomes abundant may influence ecosystem function. The consequent altered timing of food and carbon availability for other higher trophic levels (e.g. zooplankton) can have wider food web impacts and the sedimentation of detritus (e.g. dead phytoplankton) can influence the microbial food web and ecosystem balance (e.g. heterotrophy-autotrophy) and the physicochemical state of the ecosystem (e.g. oxygen concentration).

A deviation from the normal seasonal cycle (such as a too high or too low biomass, or absence of some dominating phytoplankton group(s)) is indicative of an impairment of environmental status. Phytoplankton species composition changes if the amount of nutrients or the ratios of important nutrients (e.g. nitrogen and phosphorus) change, and eutrophication has resulted in more intense and frequent phytoplankton blooms in the summer.

Policy relevance of the core indicator

BSAP Segment and Objectives

MSFD Descriptors and Criteria

Primary link

Thriving and balanced communities of plants and animals

D4 - Food webs

D4C1 The diversity (species composition and their relative abundance) of the trophic guild is not adversely affected due to anthropogenic pressures.

Secondary link

Natural Distribution and occurrence of plans and animals

D5 - Eutrophication

D5C3 The number, spatial extent and duration of harmful algal bloom events are not at levels that indicate adverse effects of nutrient enrichment.